As is well known, fuel cell automobiles obtain electric power using hydrogen and oxygen as fuels. They have attracted attention as the next generation of clean automobiles which do not discharge carbon dioxide (CO2) or harmful substances such as nitrogen oxides (NOX) or sulfur oxides (SOX) as do conventional gasoline-powered or diesel-powered automobiles. In Japan, under the guidance of the Ministry of Economy, Trade, and Industry, it is planned to introduce 5 million vehicles by the year 2020.
At present, the biggest problem with respect to fuel cell automobiles is how to realize the practical generation and storage of hydrogen as a fuel, and various types of research and development are being pursued.
Conventional methods thereof include a method in which a hydrogen gas cylinder is directly mounted on a vehicle, a method in which methanol or gasoline is reformed to obtain hydrogen by a reformer mounted on a vehicle, and a method in which a hydrogen-storing alloy which can absorb hydrogen is mounted on a vehicle.
Each of these methods has strengths and weaknesses, but in Japan, in December of 2002, the first fuel cell automobile in the world having a hydrogen gas cylinder mounted thereon was sold, and a number of the vehicles are already being used as government vehicles by the Ministry of Land, Infrastructure, and Transport, for example.
However, although present fuel cell automobiles have a maximum speed of approximately 150 km per hour and an output of approximately 100 horsepower which is close to the performance of a gasoline-powered automobile used as a private vehicle, due to limitations on the size of gas cylinders, the distance for which they can continuously run is at most only 300 km, and this is an impediment towards their general use.
At present, an increasing amount of research and development is being carried out for promoting the spread of fuel cell automobiles as the next generation of clean automobile by improving and lowering the cost of fuel cell automobiles having a high pressure hydrogen gas cylinder mounted thereon. To achieve these goals, it is necessary to overcome the following problems.
Namely, there are problems such as lengthening the continuous running distance, providing infrastructure such as gas stations, and developing safer technology for hydrogen.
It is calculated that in order to increase the running distance to 500 km, for example, it is necessary to increase the pressure of hydrogen in a vehicle-mounted gas cylinder from the present value of 35 MPa to 70 MPa. In addition, instead of existing gasoline stations, hydrogen gas stations will be necessary. As a result, it will be necessary to provide for the generation, transport, and storage of high pressure hydrogen gas and rapid filling thereof (supply to vehicles).
Since hydrogen gas is flammable, it is necessary to exercise special care when handling it. However, there are many unknown matters concerning the interaction of ultrahigh pressure hydrogen gas exceeding 50 MPa and the structure of components of equipment, and there is a strong desire for establishment of technology for its safe utilization.
The fuel cell automobiles which were sold last year used already existing SUS316 austenitic stainless steel, the soundness of which is already widely recognized. This is because its susceptibility to hydrogen gas embrittlement in a hydrogen gas environment of up to about 35 MPa is better compared to other structural steels (such as STS480 (JIS G 3455) low carbon steel or SUS304 stainless steel) and because techniques for working and welding it are already established.
However, in order to use SUS316 steel under a hydrogen gas pressure increased from 35 MPa to 70 MPa, piping which conventionally had an outer diameter of 26.2 mm and an inner diameter of 20 mm (a pipe wall thickness of 3.1 mm) must be changed to piping having an outer diameter of 34.7 mm and an inner diameter of 20 mm (pipe wall thickness of 7.35 mm), since the conventional material does not have sufficient strength unless its wall thickness is at least doubled and its weight is at least 3 times as large. Therefore, a large increase in the weight mounted on a vehicle and an increase in the size of hydrogen gas stations are unavoidable. These are great impediments to its practical application.
As disclosed in Japanese Published Unexamined Patent Applications Hei 5-98391, Hei 5-65601, Hei 7-216453, and Hei 7-26350, for example, it is generally known that the strength of usual austenitic stainless steel can be improved by cold working, and that it is possible to increase the strength and reduce the wall thickness of a pipe by drawing or by rolling.
FIG. 1 is a graph showing a typical relationship between the degree of cold working (percent reduction in cross section) and tensile strength. It can be seen therefrom that a high strength can be realized by increasing the degree of cold working.